In recent years, a vertical cavity surface emitting laser (VCSEL, hereinafter simply referred to as “surface emitting laser”), in which light resonates in a vertical direction with respect to a substrate surface as indicated by the name, has been attracting attentions as a light source for communication like optical interconnection.
Compared with a conventional edge emitting laser, the surface emitting laser has advantages that: a two-dimensional array of the lasers can be easily formed; it is unnecessary to cleave the element to form a mirror unlike the edge emitting laser; laser oscillation is possible with an extremely low threshold value because a volume of an active layer is considerably small; and power consumption is low.
Since the surface emitting laser has an extremely short cavity length of about one wavelength due to the inherent structure, the surface emitting laser has a characteristic that a basic mode oscillation is naturally obtained for an oscillation spectrum. Therefore, the surface emitting diode maintains a single longitudinal mode easier than the edge emitting laser, such as a distributed feedback (DFB) laser. In addition, the surface emitting laser attracts attentions as a laser essentially more suitable for optical communication than the DFB laser or the like because, for example, it is possible to obtain a narrow far field pattern (FFP) and a relatively low intensity noise.
With such advantages, the surface emitting laser attracts attentions as a signal light source in an optical communication network and optical interconnection that transmits information by optically connecting computers and as a device for other various applications.
FIG. 7 is a perspective sectional view of a conventional surface emitting laser. FIG. 8 is an explanatory diagram for explaining structures of a lower semiconductor multilayer mirror and an upper semiconductor multilayer mirror. Note that, portions common to FIG. 7 and FIG. 8 are denoted by identical reference numerals. To manufacture a surface emitting laser 100 shown in FIG. 7, first, a lower semiconductor multilayer mirror (lower distributed bragg reflector (DBR) mirror) 112 is formed on an n-type GaAs substrate 11 by a metal organic chemical vapor deposition (MOCVD) method. As shown in FIG. 8, in the lower semiconductor multilayer mirror 112, a stacked structure of an n-type high-refractive-index area 141 and an n-type low-refractive-index area 142 having respective thicknesses of λ/4n (λ is an oscillation wavelength and n is a refractive index) forms one pair, and for example, thirty-five pairs are stacked. The n-type high-refractive-index area 141 is formed of, for example, n-type Al0.2Ga0.8As, and the n-type low-refractive-index area 142 is formed of, for example, n-type Al0.9Ga0.1As.
Then, a quantum well (QW) active layer 32 vertically sandwiched by cladding layers 31 and 33 is formed on the lower semiconductor multilayer mirror 112. Further, an AlzGa1-zAs (0.95≦z≦1) layer 15 for forming a current confinement layer in a later process is formed. Usually, AlAs is used as the current confinement layer. Moreover, an upper semiconductor multilayer mirror 116 (upper DBR mirror) is formed on the AlzGa1-zAs (0.95≦z≦1) layer 15. Here, as shown in FIG. 8, in the upper semiconductor multilayer mirror 116, assuming that a stacked structure of a p-type high-refractive-index area 145 and a p-type low-refractive-index area 146 having respective thicknesses of λ/4n (λ is an oscillation wavelength and n is a refractive index) forms one pair, for example, twenty-five pairs are stacked. The p-type high-refractive-index area 145 is formed of, for example, p-type Al0.2Ga0.8As, and the p-type low-refractive-index area 146 is formed of, for example, p-type Al0.9Ga0.1As. In addition, a p-type GaAs contact layer 17 is formed on the upper semiconductor multilayer mirror 116.
Next, an outer edge part of a stacked structure, which consists of the upper semiconductor multilayer mirror 116, the AlAs layer 15, the cladding layer 33, the QW active layer 32, the cladding layer 31, and a part of the lower semiconductor multilayer mirror 112 is removed by a photolithography process and an etching process (dry etching or wet etching). Consequently, for example, a columnar mesa-post with a diameter of 30 micrometers is formed.
Next, oxidation treatment is performed at temperature of about 400° C. in a moisture vapor atmosphere to selectively oxidize the AlzGa1-zAs (0.95≦z≦1) layer 15 from a sidewall of the mesa-post and form an Al oxide layer 14. For example, when a diameter of the mesa-post is 30 micrometers and the Al oxide layer 14 is formed in a ring shape with a band width of 10 micrometers, an area of the AlzGa1-zAs (0.95≦z≦1) layer 15 in the center, that is, an area of an aperture to which a current is injected is about 80 μm2 (with a diameter of 10 micrometers).
Then, a silicon nitride film 19 functioning as a protective layer is formed on an upper surface and a side surface of the mesa-post and an exposed upper surface of the lower semiconductor multilayer mirror 112. Subsequently, periphery of the mesa-post, on which the silicon nitride film 19 is formed, is filled with polyimide 22. The silicon nitride film 19 formed on the upper surface of the mesa-post is removed in a circular shape with a diameter of 30 micrometers to further form a p-type electrode 18 of a ring shape with an inner diameter of 20 micrometers and an outer diameter of 30 micrometers on the p-type GaAs contact layer 17 exposed by the removal. After grinding the substrate to have a thickness of, for example, 200 micrometers, an n-type electrode 21 is formed on the back of the n-type GaAs substrate 11. An electrode pad 20, on which a wire is bonded, is formed on the polyimide 22 to come into contact with the p-type electrode 18.
The characteristic in the structure explained above is that the AlzGa1-zAs (0.95≦z≦1) layer 15 with a resistance lower than that of the surrounding Al oxide layer 14 is arranged on a central part of the QW active layer 32. This AlzGa1-zAs (0.95≦z≦1) layer 15 makes it possible to flow a current intensively only in a narrow part of the active layer 13. Such a structure is called an oxidation confinement type surface emitting laser. Laser characteristics like a laser oscillation threshold value are improved significantly.
In the surface emitting laser, the current confinement structure is important. In addition, from the viewpoint of selection of an oscillation wavelength, improvement of a thermal conductivity, and the like, structures of the lower semiconductor multilayer mirror 112 and the upper semiconductor multilayer mirror 116 vertically sandwiching the active layer 13 are also very important. It is known that, in the lower semiconductor multilayer mirror 112 and the upper semiconductor multilayer mirror 116, a refractive index difference increases as a difference of Al composition between a high-refractive-index area and a low-refractive-index area increases, and a satisfactory reflectivity is obtained. In addition, it is also known that the thermal conductivity increases as the Al composition difference increases (Afromowitz M A et al, Journal of Applied Physics 44, pp 1292, 1973). If the reflectivity is large, the number of pairs of semiconductor multilayer mirrors can be reduced. In addition, if the thermal conductivity is large, a surface emitting laser, which has satisfactory thermal saturation characteristics of optical output power and operates stably at high power even in a high-temperature operation environment, can be manufactured.
However, to obtain a large refractive index difference and a high thermal conductivity, if a composition y of an AlyGa1-yAs layer (x<y<1), which is a low-refractive-index area of any one of a lower semiconductor multilayer mirror and an upper semiconductor multilayer mirror or both, is set close to 1, a state in which the low-refractive-index area is easily oxidized is created. In particular, when the composition y is set too large in the upper semiconductor multilayer mirror 116, if oxidation treatment is performed in a moisture vapor atmosphere to obtain the Al oxide layer 14, the AlyGa1-yAs layer (x<y<1), which is the low-refractive-index area of the upper semiconductor multilayer mirror 116, may be oxidized together with the AlzGa1-zAs (0.95≦z≦1) layer 15. When a volume of an oxide film increases in the lower semiconductor multilayer mirror 112 or the upper semiconductor multilayer mirror 116, characteristics deteriorate, for example, an oscillation threshold value increases and dislocation occurs often.
As a background art of the invention, “Optoelectronics semiconductor device with mesa” disclosed in U.S. Pat. No. 5,408,105 is characterized in that an entire lower semiconductor multilayer mirror is used as an AlAs mirror layer, and a lower semiconductor multilayer film is not etched.
Incidentally, when a surface emitting laser is used as a signal light source, a surface emitting laser, which has an emission wavelength of 0.8 micrometer to 1.65 micrometers including a low-loss waveband of an optical fiber serving as a transmission medium, is required. In surface emitting lasers in this wavelength band, for a long time, it has been impossible to realize a surface emitting laser, which oscillates a laser beam having a long wavelength, for example, a wavelength of 1.2 micrometers or more, due to difficulty in crystal growth. However, recently, a surface emitting laser, which oscillates a laser beam having a wavelength of 1.2 micrometers to 1.3 micrometers, has been realized by the inventors (Japanese Patent Application Laid-Open No. 2001-124300).
FIG. 19 shows a structure of the surface emitting laser described in Japanese Patent Application Laid-Open No. 2001-124300. This surface emitting laser has a structure in which a buffer layer 1102, a lower reflective layer 1103, a lower cladding layer 1104, an active layer including a QW layer 1105, and an upper cladding layer 1106 are sequentially stacked on a substrate 1101. Further, the surface emitting laser has a stacked structure of a current confinement layer 1108 processed in a mesa shape, an upper reflective layer 1109, and a contact layer 1110 on the upper cladding layer 1106. The current confinement layer 1108 is formed of a current injection area 1107a consisting of an AlAs layer in a central part and a selectively oxidized area 1107b formed by selectively oxidizing an end of the AlAs layer. In addition, an n side electrode 1114 is arranged on a lower surface of the substrate 1101. Then, in the active layer including the QW layer 1105, by adding a small amount of Sb in GaInNAs forming the QW layer, a crystallographic quality of the active layer including the QW layer 1105 is improved. In this way, recently, laser oscillation of a surface emitting laser in a 1.3-micrometer-band has been performed utilizing the improvement in a structure of a QW layer and a selective oxidation technique of an AlAs layer.
To use a surface emitting laser as a signal light source in an optical communication network, it is necessary to realize a surface emitting laser that emits a laser beam having a wavelength with a low loss when the laser beam is transmitted through an optical fiber for transmission and having a fixed intensity. Therefore, a surface emitting laser having an emission wavelength of 1.2 micrometers or more has been developed, and an example of realizing laser oscillation using a GaInNAs material for an active layer has been reported according to the progress of a crystal growth technique in recent years.
For example, in the Post Deadline Paper (PD1.2) of the LEOS-2001 Annual Meeting, the group of Agilent Technologies Laboratories reported about a surface emitting laser of an oxidation confinement type. According to this report, there is a surface emitting laser that has a lower semiconductor multilayer mirror in which forty layers of n type DBR mirror are stacked sequentially, an active layer including a triple QW layer formed of GaInNAs, and an upper semiconductor multilayer mirror in which twenty-eight layers of a p type DBR mirror and includes an opening portion with a diameter of 11 micrometers by arranging a current confinement layer in a part of the p-type upper semiconductor multilayer mirror. With such a structure, continuous oscillation at a room temperature is realized, and a surface emitting laser with a threshold current of about 6 milliamperes and maximum optical output power of about 0.7 milliwatt is realized.
FIG. 35 is a perspective sectional view of the conventional surface emitting laser. FIG. 36 is an explanatory diagram for explaining structures of a lower semiconductor multilayer mirror and an upper semiconductor multilayer mirror. Note that, portions common to FIG. 35 and FIG. 36 are denoted by identical reference numerals. To manufacture a surface emitting laser 3100 shown in FIG. 35, first, a lower semiconductor multilayer mirror (lower DBR mirror) 3112 is formed on an n-type GaAs substrate 3111 by an MOCVD method. Here, as shown in FIG. 36, in the lower semiconductor multilayer mirror 3112, assuming that a stacked structure of an n-type high-refractive-index area 3141 and an n-type low-refractive-index area 3142 having respective thicknesses of λ/4n (λ is an oscillation wavelength and n is a refractive index) forms one pair, for example, thirty-five pairs are stacked. The n-type high-refractive-index area 3141 is formed of, for example, n-type GaAs, and the n-type low-refractive-index area 3142 is formed of, for example, n-type Al0.9Ga0.1As.
However, “Optoelectronics semiconductor device with mesa” disclosed in U.S. Pat. No. 5,408,105 also has a problem in that etching accuracy has to be extremely strict.
In addition, there are problems that should be solved in using a surface emitting laser for an application like a signal light source. First, it is necessary to unify lateral modes of an oscillating laser beam. When a mode higher in a lateral direction is present in the lateral modes, this causes marked deterioration in a signal waveform in proportion to a transmission distance at the time of optical transmission, in particular, at the time of high-speed modulation. Therefore, it is necessary to realize single lateral mode oscillation to realize long distance transmission.
In a surface emitting laser, it is naturally difficult to stabilize lateral modes due to a structure thereof. Therefore, in a surface emitting laser including selectively oxidized areas, single lateral mode oscillation is realized by adjusting a diameter of a current injection area sandwiched by the selectively oxidized areas. However, conventionally, it is difficult from the viewpoint of controllability to realize the single lateral mode oscillation by adjusting only the diameter of the current injection area in a surface emitting laser in a 1300-nanometer-band (in a range of about 1260 nanometers to 1360 nanometers).
In addition, even if the single lateral mode oscillation can be realized, when a value of a threshold current increases, a problem like an increase in power consumption is caused. Therefore, it is necessary to realize the single lateral mode oscillation while controlling the increase in a value of a threshold current. For this purpose, for example, it is necessary to set a diameter of a current injection layer to, for example, φ5 micrometers, which is disadvantageous from the viewpoint of a working voltage and optical output power. Moreover, reliability of the surface emitting laser has to be secured. This is because the surface emitting laser is required to have sufficient reliability to use the surface emitting leaser element for a signal light source or the like.
Moreover, when the surface emitting laser is used for a signal light source or the like, it is necessary that direct modulation is possible at a level of 10 Gbit/s. This is a numerical value necessary for actually using the surface emitting laser as a signal light source according to an increase in a channel capacity in recent years.
When the surface emitting laser reported by the group of Agilent Technologies Laboratories is actually used as a signal light source, a new problem occurs. Since a signal beam is transmitted in a long distance in an optical communication system, in general, a laser beam outputted from a signal light source is required to have a light intensity of about 1 milliwatt at the minimum. Since a maximum light intensity of the surface emitting laser is only about 0.7 milliwatt, it is inappropriate to use the surface emitting laser as a signal light source at the present point.
To directly modulate a laser beam at 2.4 GBit/s or more, for example, 10 GBit/s, in general, it is necessary to drive the surface emitting laser with an injection current five times as large as a threshold current. In the case of the surface emitting laser, since the threshold current is 6 milliamperes, the injection current at the time of driving is 30 milliampere or more. Thus, it is unrealistic to use the surface emitting laser in terms of power consumption and taking into account the fact that thermal saturation occurs actually. To use the surface emitting laser as a signal light source, it is desirable that the threshold current is about 1 milliampere and the injection current at the time of driving is about 5 milliamperes to 6 milliamperes. To realize the light intensity of 1 milliwatt when the injection current is 5 milliamperes, it is necessary to set slope efficiency to 0.25 mW/mA, and when the injection current is 6 milliampere, it is necessary to set slope efficiency to 0.2 mW/mA. Thus, it is inappropriate to use the surface emitting laser as a signal light source from the viewpoint of a slope efficiency as well.
Moreover, in the surface emitting laser in the 1300-nanometer-band (1260 nanometers to 1360 nanometers) oscillation under the present situation, since crystal growth is difficult for any of the above-mentioned active layers, a low oscillation threshold value and a high slope efficiency cannot be realized. In particular, in the surface emitting laser, oscillation by direct modulation is stable in a high frequency band. The surface laser element is advantageous in this respect compared with the edge-emitting laser like a distributed DFB laser. However, a new problem occurs if it is attempted to realize oscillation with a wavelength longer than VCSEL in a 0.85 micrometer to 0.98-micrometer-band like 1.2 micrometers to 1.3 micrometers in the surface emitting laser. More specifically, laser oscillation is made unstable due to inter-valence-band absorption or free carrier absorption in a semiconductor multilayer mirror. In the present situation, satisfactory characteristics are not realized even in serial transmission in 10 kilometers to 20 kilometers with direct modulation at about 10 Gbps.
The invention has been devised in view of the drawbacks of the conventional technique, and it is an object of the invention to provide a surface emitting laser with an improved reflectivity and temperature characteristics by causing an AlAs layer to be present inside a semiconductor multilayer mirror, which is not oxidized easily, according to film thickness control of the AlAs layer rather than controlling oxidation speed according to a difference of composition of Al as in the conventional technique.
The invention has been devised in view of the drawbacks of the conventional technique, and it is another object of the invention to provide a surface emitting laser that has a lower threshold current and is highly reliable and with which single lateral mode oscillation is possible and direct modulation is possible, and a transceiver, an optical transceiver, and an optical communication system using the surface emitting element.
The invention has been devised in view of the drawbacks of the conventional technique, and it is still another object of the invention to provide a surface emitting laser with which a threshold current is controlled to be about 1 milliampere and slope efficiency is 0.2 mW/mA or more, and an optical transceiver, an optical communication device, and an optical communication system using the surface emitting laser.
The invention has been devised in view of the drawbacks of the conventional technique, and it is still another object of the invention to provide a surface emitting laser of a structure having a long wavelength band of 1.2 micrometers or more as an oscillating wavelength, which can realize a low oscillation threshold value, high slope efficiency, and high frequency direct modulation by reducing an absorption loss due to a p-type semiconductor reflector, and a transceiver, an optical transceiver, and an optical communication system using the surface emitting laser.